1. Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to an air cooled turbine rotor blade.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A gas turbine engine, such as an industrial gas turbine (IGT) engine, includes a turbine with multiple rows or stages or stator vanes that guide a high temperature gas flow through adjacent rotors of rotor blades to produce mechanical power and drive a bypass fan, in the case of an aero engine, or an electric generator, in the case of an IGT. In both cases, the turbine is also used to drive the compressor.
It is well known in the art of gas turbine engine design that the efficiency of the engine can be increased by passing a higher gas flow temperature through the turbine. However, the turbine inlet temperature is limited by the material properties of the turbine, especially for the first stage airfoils since these are exposed to the highest temperature gas flow. As the gas flow passes through the various stages of the turbine, the temperature decreases as the energy is extracted by the rotor blades.
Another method of increases the turbine inlet temperature is to provide more effective cooling of the airfoils. Complex internal and external cooling circuits or designs have been proposed using a combination of internal convection and impingement cooling along with external film cooling to transfer heat away from the metal and form a layer of protective air to limit thermal heat transfer to the metal airfoil surface. However, since the pressurized air used for the airfoil cooling is bled off from the compressor, this bleed off air decreases the efficiency of the engine because the work required to compress the air is not used for power production. It is therefore wasted energy as far as producing useful work in the turbine.
One method of maintaining a relatively low metal temperature for a turbine airfoil is to use a thin outer airfoil surface so that the heat transfer rate from the hot outer surface to the cooler inner surface is high. With a thin airfoil wall and a high heat transfer rate, the outer airfoil surface will have a lower metal temperature than would a relatively thick airfoil wall. A thicker wall is desired in order to provide for a rigid and structurally sound airfoil. Thin airfoil walls require support. An airfoil wall with a lower metal temperature requires less cooling air flow and thus will improve the turbine efficiency.
The U.S. Pat. No. 5,702,232 issued to Moore on Dec. 30, 1997 and entitled COOLED AIRFOIL FOR A GAS TURBINE ENGINE discloses an airfoil with a near wall cooling in the mid-chord section constructed with radial flow channels plus re-supply holes in conjunction with film discharge cooling holes, and is shown in FIG. 1. In the Moore design, the spanwise (radial direction) and chordwise (perpendicular to the radial direction) cooling flow control due to the airfoil external hot gas temperature and pressure variation is difficult to achieve. This is important since some surfaces of the outer airfoil are at higher metal temperatures than other surfaces, and thus require more cooling to control the overall metal temperature of the airfoil. In addition, a single radial channel flow is not the best method of utilizing the cooling air resulting in a low convective cooling effectiveness. The dimension for the airfoil external wall has to fulfill the casting requirement. an increase in the conduction path will reduce the thermal efficiency for the blade mid-chord section cooling. The blade leading edge and trailing edge sections are cooled with conventional cooling methods.